Generally, a secondary battery is a rechargeable battery, different from a primary battery which cannot be charged, and the secondary battery is widely used for electronic devices such as cellular phones, notebooks and camcorders, and electric vehicles (EV). In particular, a lithium secondary battery with an operating voltage of about 3.6V tends to be used more and more due to a greater capacity in comparison to a nickel-cadmium battery or a nickel-hydrogen battery frequently used as a power source of an electronic device, and a high energy density per unit weight.
A lithium secondary battery generally uses a lithium-based oxide and a carbon material as a positive electrode active material and a negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate respectively coated with the positive electrode active material and the negative electrode active material are disposed with a separator being interposed therebetween, and an exterior material, namely a battery case, configured to accommodate and seal the electrode assembly together with an electrolyte. The lithium secondary battery may be classified into a can-type secondary battery in which an electrode assembly is included in a metal can and a pouch-type secondary battery in which an electrode assembly is included in a pouch made of an aluminum lamination sheet, depending on the shape of the battery case.
The pouch-type secondary battery is classified into a unidirectional battery in which electrode leads connected to positive/negative electrode tabs of a secondary battery are exposed at one side and a bidirectional battery in which the electrode leads are exposed oppositely. Here, the bidirectional battery is configured as shown in FIG. 1.
The pouch-type secondary battery 10 depicted in FIG. 1 includes an electrode assembly 20, electrode leads 30, 40, and a pouch-type battery case 50. Reference symbol “55” represents a sealing portion thermally bonded.
The battery case 50 has a laminate structure of an outer resin layer, a metal layer and an inner resin layer, so that both side portions, an upper end portion and a lower end portion where upper and lower pouches contact each other may be bonded to each other by heating and pressing them to fuse resin layers to each other. Both side portions may be uniformly sealed by means of fusing since the same resin layers of the upper and lower pouches come into direct contact with each other. Meanwhile, since the electrode leads 30, 40 protrude at the upper end portion and the lower end portion, the upper end portion and the lower end portion are thermally bonded in a state where a film-type sealing material is interposed between the electrode leads 30, 40 to enhance a sealing property, in consideration of the thickness of the electrode leads 30, 40 and a difference in material with the battery case 50.
However, since the electrode assembly 20 of the pouch-type secondary battery 10 repeats expanding and shrinking during a charging and discharging process, the thermally bonded portions at the upper end portion and the lower end portion, particularly at both side portions, may be easily separated. If a high pressure occurs inside the battery due to degradation caused by long-term use or abnormal operation conditions, the thermally bonded sealing portion may be opened, which may result in leakage of gas, electrolyte or the like in the battery. The electrolyte is made of a combustible material and thus has a high risk of firing.
Further, even though the battery case 50 having a laminate structure is sealed by means of thermal bonding, a cutting plane of the sealing portion made of a lamination sheet (namely, a cutting plane of the sheet) is exposed outwards, and thus external moisture, air or the like may penetrate into the secondary battery through the sealing portion.
A moisture penetration rate has a relation with time and temperature and is increasing if temperature and humidity at an environment around the secondary battery are higher. Moisture or the like penetrating through the cutting plane of the sealing portion made of a lamination sheet reacts with the electrolyte to form gas and also promote degradation of the secondary battery by means of destruction of the positive electrode material or the like, which is not desirable in view of safety and life characteristics. In other words, the pouch-type secondary battery currently used in the art has a weak structure against external environments since the cutting plane of the sealing portion of the pouch is exposed outwards.
To solve this problem, a sealing width may be designed wider to protect the secondary battery, which however is contradictory to the current trend of the related art, which is directed to higher density, and thus it is required to design the sealing width as small as possible in order to enhance space utilization of the secondary battery.